Adsorption Of NO Molecules Research Articles

Adsorption, desorption and reaction kinetics of nitric oxide on a stepped pd(111) surface

We performed LEED, TDMS and molecular beam measurements on the adsorption system NO/stepped Pd(111) down to 160 K. The initial sticking coefficient is found to be unity independent of temperature, and the adsorption kinetic is dominated by a precursor state. The saturation coverage at temperatures below 200 K is one monolayer, which is correlated with a sharp (2 × 2) LEED pattern. At temperatures above 400 K, a small amount of the adsorbed NO molecules dissociates, which occurs at structural defects, i.e. at steps, only. Besides NO the only desorbing species are N 2 and N 2O. The dissociation capability of the surface is extremely sensitive to adsorbed oxygen, which blocks the step sites. From thermal desorption measurements we could determine the fraction of NO molecules that dissociate as a function of the applied heating rate; this allowed an estimate of the kinetic parameters. The activation energy for dissociation was thus found to be 1.2 eV with a pre-exponential of 4 × 10 11 s −1. Since dissociation could be suppressed using a small pre-coverage of oxygen in thermal desorption experiments with NO we were able to determine the desorption kinetics from an evaluation of the desorption spectra via isothermic and isosteric desorption rates: in the low coverage limit the activation energy for desorption is 1.52 eV with a pre-exponential of 2.6 × 10 14 s −1. The variation of the desorption energy with coverage indicates the operation of strong lateral interactions between the adsorbed NO molecules.

Structure sensitivity of NO adsorption on a smooth and stepped Pt(100) surface

The adsorption of NO was studied in the temperature range 300–500 K by low-energy electron diffraction (LEED), photoemission, and Auger electron spectroscopy on a smooth Pt(100) crystal exhibiting a 1 × 1, 5 × 20, or √29 × √170 structure, and a stepped Pt [4(100) × (111)] crystal with 1 × 1 or reconstructed surface. The initial sticking coefficient of NO on all surfaces was about 0.6 at 300 K. Adsorbed molecular NO on the Pt(100)−l × 1 surface gave rise to a sharp c(2 × 4) type LEED pattern. A structure model for this pattern was proposed on the basis of nonlinearly bonded NO molecules. On heating the adsorbed NO molecules dissociated on all surfaces; the onset of dissociation was approximately at 400 K independent of structure and steps. No adsorption of NO could be observed on reconstructed surfaces at T > 380 K. On the other hand, unreconstructed surfaces were able to adsorb NO molecularly at temperatures up to 410 K and dissociatively at T = 423 and 435 K for the smooth and stepped surface, respectively. Dissociative NO adsorption was also verified for a carbon stabilized Pt(100)−1 × 1 surface at 473 K. We conclude that surface reconstruction can play a more significant role on NO adsorption kinetics than monatomic steps.

Crystal field surface orbital-bond energy bond order (CFSO-BEBO) calculations for surface reactions: I. The reactions CO + O 2, NO + CO and H 2 + O 2 on a platinum (111) surface

The crystal field surface orbital-bond energy bond order (CFSO-BEBO) model of chemisorption has been extended in order to make predictions of activation energies of surface reactions. The reactions of O 2 with CO, NO with CO, and H 2 with O 2 on a Pt (111) surface are considered. The model allows a choice to be made between the Langmuir-Hinshelwood and Rideal-Eley mechanisms of surface reactions based on calculated activation energies. For each of the three reactions investigated the Rideal-Eley mechanism has the lowest activation energy. The H 2 and O 2 reaction is predicted to have zero activation energy, and the O 2 and CO reaction is predicted to have an activation energy of 16 kcal/mole. The reaction of NO and CO is predicted to proceed in the following way: The NO dissociates on the Pt surface with an activation energy of 12 kcal/mole, and the adsorbed O atom reacts with a gas phase CO molecule with an activation energy of 16 kcal/mole. The direct interaction of a gas phase CO molecule with an adsorbed NO molecule is predicted to have an activation energy of 37 kcal/mole, and this mechanism can thus be ruled out. Favorable comparisons are made between the model predictions and available experimental data.

The adsorption of nitric oxide on platinum black

Adsorption isotherms for NO chemisorption on reduced and oxidized platinum black were measured in the range from −78 to 100 °C. Monolayer coverage, attained by extrapolation of the Freundlich isotherms, yields a 1:1 correspondence between adsorbed NO molecules and Pt atoms in the surface. The chemisorption characteristics of NO on Pt show a close resemblance to those observed on transition metal oxides.

The adsorption of nitric oxide and carbon monoxide on nickel oxide

Adsorption isotherms and rates for NO and CO chemisorption were measured on supported and unsupported nickel oxide samples in the 0–140 °C temperature range. The isotherms are of the Freundlich type in the pressure range from 1 to 125 Torr. Monolayer coverage is attained at 110 Torr for the NO adsorption and at 530 Torr for the CO adsorption. The uptake at monolayer coverage in CO adsorption is ~30% smaller than the corresponding NO amount, indicating that about 1 3 of the adsorbed CO molecules are present in the bridged form. Similarly to the previously studied oxide adsorbents, there is a 1:1 correspondence between adsorbed NO molecules and Ni atoms on the surface. The chemisorption rate of both adsorbates is described by the Elovich plots. These are more complex for CO than for NO. In the investigated temperature range, the rate of chemisorption and the heat of chemisorption are higher for NO than for CO. The rates of NO chemisorption for the transition metal oxides studied so far are in the order: iron oxide > chromium oxide > nickel oxide.

A study of surface interactions on γ-alumina, silica-alumina, and silica-magnesia using the EPR spectra of adsorbed nitric oxide

The EPR spectrum of the adsorbed NO molecule was used to determine crystal and magnetic field interactions on the surface of γ-alumina, silica-alumina, and silica-magnesia. The crystal-field splitting, Δ, of the 2pπ ∗ levels of NO was found to be 0.75 eV on dehydrated γ-alumina and 0.60 eV on dehydrated silica-alumina. The spectrum on silica-magnesia indicated two adsorption sites with Δ equal to 0.60 and 0.33 eV. Spectra of NO on γ-alumina and silica-alumina show considerable hyperfine coupling with an 27Al atom at the adsorption site. The spectra were very similar to the spectrum of NO on a decationated Y zeolite. The maximum number of adsorption sites (as indicated by the spin concentration) was found to be 4(± 1) × 10 12 sites/cm 2 for 7-alumina and two types of silica-alumina. Silica-magnesia had 1.1(± 0.3) × 10 12 sites/cm 2. The number of sites and the shape of the spectra were a function of the degree of dehydration. Water added back to dehydrated γ-alumina and silica-alumina also affected the spectra. It appears that these adsorption sites are the α sites which Peri has studied using the IR spectra of adsorbed CO 2.

EPR spectra of radicals formed when NO 2 is adsorbed on magnesium oxide

Four paramagnetic species were detected by EPR when NO 2 in equilibrium with N 2O 4 was adsorbed on high surface area magnesium oxide. At pressures of NO 2 of about 10 torr, the spectrum of the adsorbed NO molecule was observed. An unidentified spectrum with a g value near 2.00 and hyperfine interaction replaced the NO spectrum at pressures near 100 torr. The characteristic spectrum of the rigid NO 2 molecule was observed at −180°C when NO 2 was adsorbed on a sample that had previously been exposed to NO 2 and degassed briefly at 300°C, or when the initial adsorption was carried out at −196°C. This spectrum is compared with the spectra of NO 2 in a number of other matrices and with the spectrum of the isoelectronic CO 2 − molecule. It appears that the adsorbed NO 2 molecule is buried with N 2O 4 in the pore structure of MgO so that little interaction with the gas phase occurs. When NO 2 was adsorbed on MgO degassed at 800°C, a species that showed an anisotropic g tensor but no hyperfine structure was also detected; the radical is tentatively identified as the NO 3 molecule.

EPR Study of NO Adsorbed on Magnesium Oxide

The EPR spectra of two paramagnetic species were observed when nitric oxide was adsorbed on high-surface-area magnesium oxide. The spectrum of one species which is interpreted as NO22− was observed at room temperature on MgO that had been degassed at 800°C. It was characterized by gxx=gyy=2.0068, gzz=2.0025, and axx≃ayy≃0 G; for 14NO adsorbed azz=43 G while for 15NO adsorbed azz=61 G. The other radical is identified as the adsorbed NO molecule in a strong field at the surface such that the spin—orbit coupling is partially quenched. Its spectrum was observed only at low temperatures (−180°), and was characterized by gxx=gyy=1.995 with gzz=1.90 for MgO degassed at 800° or at 300°C. For 14NO, axx=33 G and azz<10 G while for 15NO, axx=47 G and azz<10 G. Theoretical predictions as well as the spectra of the isoelectronic N2− molecule—ion are in accord with the assignment of this spectrum. Both paramagnetic species could be partially removed by degassing the sample at room temperature, notwithstanding the rather strong electronic interactions which are indicated by the spectra.

Chemisorption of Nitric Oxide on Tungsten

The chemisorption of NO on polycrystalline W has been investigated using a combination of ultrahigh-vacuum techniques: flash-desorption mass spectroscopy and field-emission microscopy. It has been found that NO chemisorbs nondissociatively on W at room temperature. Dissociation occurs at elevated temperature with an activation energy of about 47 kcal/mole (197 kJ/mole). Kinetic evidence indicates that dissociation involves the interaction of an adsorbed NO molecule with a neighbor empty site. At high NO coverage, dissociation is rate controlling for the liberation of ω-mode nitrogen near 1000°K. Heating of an NO-shadowed W field-emission tip above ∼700°K results in the formation of a receding boundary which remains sharp during the entire course of its backward motion over the initially covered region. This behavior is consistent with a model involving dissociation of NO at the boundary where empty sites are available. For monolayer NO (1.4×1015 molecules cm−2) on polycrystalline W, Δφ=+1.85±0.10 eV; this corresponds to an average surface dipole moment of 0.35-D per chemisorbed NO molecule at full coverage.

On a new alkali-metal

The aim of this article is to investigate the sensing performance of NO gas molecule on the graphene nanoribbon domain for the determination of structural and electronic properties. Effect of an alkali metal (lithium) and a transition metal (iron) on the armchair oriented graphene nanoribbon (ArGNR) surface for the sensing purpose of NO gas has been performed through the quantum mechanics based Density Functional Theory (DFT) calculations. Various configurations of ArGNR doped with Li and Fe atoms such as one-edge doped, center doped, both-edge doped Li-ArGNR and Fe-ArGNR have been simulated, and a detailed comparative study of lithium and iron doping on different configurations of ArGNRs for the adsorption energy, stability analysis, band gap analysis and density of states analysis has been quantitatively evaluated. By comparing the adsorption energy of NO, it is found that Li doping enhances the strength of NO adsorption on the different variants of ArGNR. Computational results predict that the undoped ArGNR is insensitive to the NO gas adsorption with adsorption energy of about −0.41 eV. Our results determine that substitutional doping of Li doping at one edge doped and both-edge doped position increases the adsorption abilities of ArGNRs in these configurations with adsorption energies of approximately −6.92 eV and −9.64 eV that is 16 and 23 times greater than the pristine ArGNR (Pr-ArGNR). Band nature for both type of doping estimates the changing behavior of ArGNRs from semiconductor to metallic transition after the adsorption of NO molecule. It is concluded that the Li doping at one edge and both edge position of ArGNR makes it an excellent potential sensing material for the sensing purpose of NO gas as compared to the Fe doped configurations.